The soil of a cropped field has field capacity of 25% and wilting point of 13% on weight basis. The effective root-zone depth of the crop is 0.70 m and the consumptive use of water by the crop is 5 mm day⁻¹. Apparent specific gravity of the soil is 1.50. If the allowable soil moisture depletion is 40%, the permissible moisture depletion between irrigations and the frequency of irrigation are
Options:
(A) 5 cm; 10 days
(B) 10 cm; 7 days
(C) 8 cm; 12 days
(D) 4 cm; 8 days
Official Answer Key:
A
Step-by-Step Solution & Explanation
• Total available water (weight basis) = FC – WP = 25 – 13 = 12% by weight.
• Depth of available water in root zone = (FC–WP)/100 × apparent specific gravity × root depth = 0.12 × 1.50 × 700 mm = 126 mm.
• Permissible (allowable) depletion = 40% of available water = 0.40 × 126 = 50.4 mm ≈ 5.0 cm.
• Irrigation frequency = permissible depletion / consumptive use rate = 50.4 mm / 5 mm/day ≈ 10.1 ≈ 10 days.
• Final Answer: (A) 5 cm; 10 days
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